1. Field of the Invention
The present invention relates to a timing device such as a timing disk or a timing ruler which consists of a carrier having a first group of code markings disposed in at least one code track, which group of code markings is scanned by at least one sensor unit for producing a digital signal.
2. Description of the Related Art
A timing disk or a timing ruler of the aforedescribed type is described, for example, in U.S. Pat. No. 5,508,088 (common assignee""s PWB basic patent). According to an embodiment of the device described therein, three concentric code tracks are arranged on a timing disk, with each of the code tracks having different markings. Three sensor units are required for scanning the three tracks, with the sensor units arranged in a similar manner as those of FIG. 1a, 1b or 2a, 2b of the known device. Accordingly, to scan several tracks disposed on a timing disk, several sensor units are arranged side-by-side in the radial direction, which increases the overall size of the scanning unit. In addition, several sensing devices require more complex electrical circuits, so that the device can more easily malfunction due to mechanical shock. The increased parts count and the more complex software introduces additional sources for errors. This is a significant disadvantage for the further improvement of printers, scanners and copiers. In practice, the carrier also ages which tend to make the carrier material opaque, or becomes contaminated, which is a particular problem with open systems such as inkjet printers. Both these situations can introduce errors if the sensor interprets the impurities as code markings.
It is an object of the present invention to improve the design of the timing device such as timing disks or timing rulers to make them more robust and less error-prone. It is another object to simultaneously capture several separate optical signals using one sensor unit through simple, material-specific and circuit-related measures. It is yet another object to provide continuous signal amplification, as used for example with potentiometer systems, and to economically produce a simple position measurement device, for example for a steering angle sensor.
These and other objects are solved with the invention by the characterizing features recited in the claims. It has been observed that two or more groups of code markings can be detected independently from one another using a single sensor unit, if the code markings of the groups have a different optical density. xe2x80x9cOptical densityxe2x80x9d refers to a gradation in gray levels between approximately 100% (dark-gray) to approximately 0% (fully transparent, fully reflective). Absolute positioning preferably is not attained by using bars, but rather by taking advantage of the increasing or decreasing gray levels. The optical density changes in this case, so that the sensor produces signals having a high intensity and corresponding to a low optical density and produces signals having a low intensity and corresponding to a high optical density, or vice versa.
The different groups of code markings in a track can be scanned simultaneously using a two-channel or a multixe2x80x94channel sensor unit. The groups of code markings may also overlap one another. It is sufficient if the sensor unit senses a noticeable change in the optical density, which causes a corresponding change in the voltage captured by the sensor unit. Voltage differences of, for example, 100 xcexcV have proven to be adequate; however, other voltage differences can be used depending to the sensitivity of the measurement devices.
Suitable sensor units consist of an LED or another light source and of photo transistors or other light sensitive scanning devices. For controlling start and/or end positions or for calibration purposes, these sensor units can detect signals with either a constant separation or an arbitrary separation over the entire segment ranges of the timing disk or the timing ruler.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.